Larvae outbreaks usually hit at the tip of the Whangaparaoa Peninsula and travel down, she says.

"But I think it might have gone across the entire peninsula now.

"It is pretty nasty because it lasts for a couple weeks."

Adults and children from Army Bay to Manly, Red Beach and Orewa have experienced rashes.

Bathers may have a tingling sensation while in the water, and a rash with small blisters develops during the next few hours.

On top of itching, some people feel unwell with symptoms such as fever, nausea, abdominal pain and diarrhoea.

Beach users should avoid the shallows or use sunscreen products with jellyfish repellent.

If people do get stung they should visit their pharmacy.

Because of the yearly occurrence the Manly chemist has developed its own lotion to deal with the sting.

"Antihistamines are also a good idea because they stop the itch," Ms Adams says.

Bathers who think they have been stung should not use a towel because rubbing will cause the larvae to release toxins.

Do not rinse with fresh water while still wearing swimwear because that will also fire up larvae stinging cells.

Shower in high-pressure water to remove any larvae still on the skin.

Swimsuits should be machine washed in hot soapy water and dried in a hot tumble dryer or in the sun. Hand-washed and air-dried swimsuits can still contain larvae which may continue to sting.

The yearly outbreaks are not a major health concern, Auckland medical officer of health Dr Simon Baker says.

"No-one is going to die of being stung - it is basically just annoying."

Some people won't be affected at all, but those with sensitive skin and children will be the worst affected, he says.

Dr Baker says the phenomena seems to be localised around the Whangaparaoa Peninsula this year.

National Institute of Water and Atmospheric Research principal scientist Dr Dennis Gordon says in spring there is an increase in plankton jellyfish feed on, which starts a sequence culminating in larger populations during late summer.